Peroxidases (EC 1.11.1) are widely found in biological systems and form a subset of oxidoreductases that reduce hydrogen peroxide (H2O2) to water in order to oxidize a large variety of aromatic compounds ranging from phenol to aromatic amines. The reaction cycle of peroxidases is quite complex and begins with activation of heme by H2O2 to form the two-electron activated Compound I (N. C. Veitch, Phytochemistry, 2004, 65, 249). Compound I is then reduced by one electron by the oxidation of the organic substrate leading to the formation of Compound II that is one electron above the resting state. The second reduction recovers the enzyme to its resting state to start a new cycle. Overall, for each molecule of hydrogen peroxide consumed, two aromatic free radicals are produced and can readily react in secondary reactions.
Peroxidases are highly sensitive to substrate inhibition, mostly by H2O2, which can lead to the formation of the reversible inactivated form of the enzyme (Compound III). Their activities are also deterred by product inhibition. Therefore, the complex kinetics associated with peroxidase enzymes can restrict their use in many processes and bioprocesses. Increasing the activities of this and other families of enzymes and their tolerance to different process conditions could improve their current use, as well as pave the way for their use in new applications.